Electrical and mechanical interconnects are essential in many computer and electrical components. Interconnects allow electrical signals such as, for example, address signals, data signals, control signals, and power and ground, to be passed from one location to another location. For example, an interconnect allows a packaged integrated circuit to receive or output electrical signals to or from an underlying printed circuit board. As is well known in the art, a packaged integrated circuit typically consists of an integrated circuit die encapsulated in a protective material such as ceramic or plastic. The packaged integrated circuit usually includes some type of electrical connector which extends from the integrated circuit to the outside of the package.
One frequently used type of prior art interconnect is a ball grid array (BGA) interconnect. In a prior art BGA interconnect, an electrically conductive sphere or ball or, more commonly, a plurality of electrically conductive balls are used to transfer signals from one location to another location. As an example, a BGA interconnect is often used to input or output electrical signals from a packaged integrated circuit to an underlying printed circuit board. In such an embodiment, a plurality of conductive balls are positioned in an array on a surface such as, for example, the bottom surface of the packaged integrated circuit. The electrically conductive balls are electrically and mechanically coupled to various input and output pads of the packaged integrated circuit. In a prior art configuration where a packaged integrated circuit is to be electrically coupled to a printed circuit board, a plurality of electrically conductive "land patterns" are arranged in a array on the top surface of the printed circuit board. The array of the electrically conductive land patterns mirrors the array of conductive balls. That is, for every conductive ball, there is a corresponding landing pattern. The electrically conductive land patterns include a conductive region referred to as a landing pad. Each landing pad is electrically connected to a capture pad. The capture pad is electrically coupled to a via which is formed into the printed circuit board. Thus, each conductive ball contacts or "lands on" a respective circular electrically conductive landing pad when the packaged integrated circuit is selectively aligned with and placed onto the printed circuit board. As a result, a signal can be passed from the integrated circuit, through a package interconnect, through a conductive ball, through a landing pad, to a capture pad, to a via, and into the underlying printed circuit board circuitry. Hence, input and output pads of the packaged integrated circuit are electrically coupled to vias in the printed circuit board.
Prior Art FIG. 1A shows a portion of printed circuit board 10 with a conventional BGA interconnect land pattern 12 disposed thereon. The conventional BGA interconnect land pattern shape is commonly referred to as "dogbone" land pattern. Prior art dogbone BGA land pattern 12 includes a capture pad 14 which is coupled to via 16 which is formed into printed circuit board 10. Dogbone BGA land pattern 12 also includes a circular landing pad 18. Circular landing pad 18 and capture pad 14 are electrically coupled by a connecting region 20. Prior Art FIG. 1B shows typical arrangement of land patterns, wherein a plurality of conventional BGA interconnect land patterns are disposed in an array.
Although dogbone BGA interconnect land patterns are well known in the art, such prior art land patterns have substantial limitations and disadvantages. For instance, as the number of vias per unit area, i.e. package density, in the printed circuit board increases, the pitch or size of prior art BGA land patterns have been correspondingly reduced. The reduced size of the prior art BGA land patterns results in several problems. For example, the small capture pads found in prior art BGA land patterns necessitate more precise drilling of vias, thereby requiring higher manufacturing accuracy. The small capture pads further require a small opening for an overlying solder mask. With such small capture pads and solder mask openings, even a slight misalignment between the solder mask and the capture pads can cause defects. Specifically, a solder mask misalignment will result in solder mask being deposited into or near the via instead of having the solder mask reside only on the periphery of the capture pad. As a result, the solder mask does not appropriately and uniformly coat the capture pad. If the via is coated with solder mask, the via will not be covered with solder. The uncoated via is then susceptible to corrosion and the like and can cause defects or shorts.
Additionally, as the density of vias in the printed circuit board increases, the distance between the landing pad and the via in prior art BGA land patterns has become extremely short. As a result, prior art BGA land patterns have limited thermal impedance between the landing pad and the via. Furthermore, it is well known that a via in a printed circuit board acts as a heat sink. Therefore, in prior art BGA land patterns, significant heat escapes or is transferred from the landing pad to the via. The heat which escapes or is transferred into the via increases the time which is required to melt solder present on the landing pad. That is, heat which is intended to melt solder on the landing pad is instead thermally transferred across the short conducting region and into the via. As a result, solder on prior art BGA landing pads can remain unmelted after convective heating process steps. The unmelted solder results in opens and/or shorts thereby leaving input and output pads of the packaged integrated circuit unconnected to the printed circuit board.
Another problem with prior art BGA land patterns is "pad lifting." Pad lifting occurs when the landing pad and/or the capture pad lifts or peels from the underlying substrate. Pad lifting may occur, for example, during replacement of a packaged integrated circuit when conductive balls are separated from respective landing pads. Pad lifting may also occur during cleaning and scrubbing of the landing pad and/or the capture pad. The problem of pad lifting is especially prevalent with small sized BGA land patterns. As prior art BGA land patterns decrease in size, less land pattern area is available to adhere to the underlying substrate. Thus, pad lifting becomes a more frequent and troublesome occurrence in smaller prior art BGA land patterns.
The creation of loose solder balls is yet another disadvantage associated with prior art BGA land patterns. A certain amount of solder is required to connect each landing pad to a respective conductive ball. Furthermore, solder paste is commonly applied in a uniform specified thickness to selected regions on, for example, the top surface of the substrate. The solder paste is commonly applied through selective aperture openings to an area or region slightly larger than the solder wettable area to which the molten solder will be confined. As the solder paste is heated, the molten solder will wick back or diffuse to the solder wettable area such as, for example, the landing pad of a prior art BGA land pattern. The wicking or diffusion of the molten solder to the solder wettable area is referred to as "pullback." As the size of prior art BGA landing pads decreases, a greater pullback is required to obtain the necessary amount of solder on the landing pad. That is, because the solder paste is applied in a uniform thickness, the area over which the solder paste is applied must extend over a much larger area than the reduced solder wettable landing pad area to which the molten solder will ultimately be confined. Thus, small prior art BGA land patterns induce significant solder pullback. As the amount of pullback increases, the chance that some portion of the solder will not pullback to the solder wettable area also increases. Portions of the solder which do not pullback to the solder wettable area can generate loose solder balls. The loose solder balls are processing defects that can cause malfunctions in the electrical environment through random shorts.
With reference still to prior art FIG. 1, wide connecting region 20 between has solder mask disposed thereon. The solder mask dam prevents solder from diffusing or wicking between landing pad 18 and capture pad 14. The size or width of connecting region 20 is yet another disadvantage of the prior art BGA land patterns. Specifically, wide connecting region 20 is susceptible to chipping or flaking. Chipping or flaking of the solder mask dam allows solder to diffuse or wick between the landing pad and the capture pad. The solder then escapes from the landing pad to the capture pad resulting in opens.
X-ray inspection of solder connections is also difficult in prior art BGA land patterns. Once a conductive ball is soldered to landing pad 18, solder extends from circular landing pad 18 to the conductive ball. An x-ray image does not clearly show the integrity or shape of the solder connection. Instead, the x-ray tends to show the circular shape of landing pad 18. Thus, in prior art BGA land patterns, x-ray inspection does not provide much insight into the integrity of solder joints.
Thus, a need exists for a BGA land pattern which reduces or eliminates defects associated with prior art BGA land patterns; a BGA land pattern which can accommodate fine pitch applications; and a BGA land pattern which does not deleteriously reduce the size of landing pads or capture pads.